Fungicide Resistance in Botrytis in Danish Strawberry Production

Abstract

A fungicide resistance survey of Botrytis populations in Danish strawberry fields was conducted in 2015 to 2019. Among 594 isolates from 46 fields under integrated pest management, 40% were resistant to fenhexamid, 76% to QoI fungicides, 57% to boscalid and 59% to anilinopyrimidines. These resistance levels were higher than in Northern Germany during the same period, reflecting the more frequent use of the corresponding commercial fungicides in Denmark. In contrast, resistance to fludioxonil (21%) was lower in Denmark than in Northern Germany. Fungicide resistance generally increased with the age of strawberry fields. Isolates with multiple resistance to the registered fungicides were more likely than sensitive isolates to possess resistance also to iprodione and thiophanate-methyl, two fungicides which were no longer used in Denmark during the sampling period. This coupling of resistance to current and old fungicides is interpreted as evidence of a differentiation of Botrytis populations into strains with enhanced and reduced competence to develop fungicide resistance. In order to retard resistance development, adjustments to Danish fungicide registration for strawberries are recommended. Further, non-chemical control methods such as removing infected fruit at harvest should be implemented.